1. Field of the Invention
The invention relates to a process for bleaching aqueous surfactant solutions using hydrogen peroxide, in a first process step the actual bleaching being carried out at alkaline pHs in the presence of inorganic additives which act as decomposition inhibitors for hydrogen peroxide and in a second process step a specific decomposition of the unreacted hydrogen peroxide and/or oxidizing secondary products formed therefrom being performed by contact with transition metals or inorganic compounds thereof.
The process is of interest in particular for surfactant solutions which contain discolorations due to their preparation, which discolorations must be bleached to give light-colored products before use in formulations for the washing and cleaning sector, including personal care applications. This relates in particular, e.g. to alkyl polyglycosides.
Alkyl polyglycosides are prepared from natural raw materials and are non-toxic and readily degradable surface-active materials. They are therefore used as washing agents and cleaning agents and as emulsifiers and dispersers. However, they only have the desired surface-active properties when the alkyl groups have at least 8 C atoms. The alkyl polyglycosides in the context of this invention comply with the formula EQU R'--O--Z.sub.n
in which R' represents a linear or branched, saturated or unsaturated aliphatic alkyl radical having 8 to 18 carbon atoms or mixtures thereof and Z.sub.n represents a polyglycosyl radical having an average value for n of 1.1 to 3 hexose or pentose units or mixtures thereof.
Preference is given to alkyl polyglycosides having alkyl radicals having 12 to 16 carbon atoms and having a polyglycosyl radical having an average value for n of 1.1 to 2. Particular preference is given to alkyl polyglycosides having a polyglycosyl radical having an average value for n of 1.1 to 1.4. The preferred polyglycosyl radical is the polyglycosyl radical.
2. Description of the Prior Art
Alkyl polyglycosides having long-chain alkyl groups are generally prepared by single-stage or multistage syntheses. A single-stage preparation process is described, inter alia, in DE-A-41 01 252.
A two-stage preparation process is specified, for example, in EP-A-0 306 652, according to which an n-butyl glycoside is first prepared by glycosidation with n-butanol and the desired long-chain alkyl polyglycoside is thereupon prepared by transglycosidation with a long-chain alcohol.
When the reaction is completed, the alkyl polyglycosides present are dissolved in long-chain alcohols. These alcohols must then be separated off if it is desired to obtain products which are completely soluble in water.
The aqueous alkyl polyglycoside solutions thus obtained are still too dark for high aesthetic requirements and must generally be subjected to a bleaching.
There are numerous references in the literature as to how the bleaching can be carried out. Thus, Staley (EP 0 165 721) describes a process for bleaching using hydrogen peroxide, sulphur dioxide, ozone or peracids. As no control or adjustment of the pH is provided, the bleaching results are unsatisfactory. Moreover, in this manner by-products can be formed to an undesired extent, some of which also have a pronounced inherent odor.
Other bleaching methods such as catalytic hydrogenation (U.S. Pat. No. 4,762,918, Staley), reaction with borohydride (EP 0 388 857, Kao) and irradiation with UV light (EP 0 526 710, H uls) have proven to be insufficiently effective.
Bleaching in the presence of bleach boosters such as alkaline earth metal ions or alkali metal silicates (WO 93 13 113, Henkel) using hydrogen peroxide does give significantly better bleaching results but it is difficult to obtain products free from hydrogen peroxide under these conditions. Residual amounts of oxidizing agents must be avoided at any rate, since they effect inter alia the decomposition of additives in alkyl polyglycoside-containing formulations. The proposal to solve the problem of residual hydrogen peroxide bleaching in the presence of transition metal compounds (DE 42 18 073, Henkel) is likewise unconvincing. The then accelerated catalytic decomposition of hydrogen peroxide to give oxygen during the bleaching reaction decreases the available amount of hydrogen peroxide. The bleaching result is thus visibly impaired and made less effective.
The object was therefore to carry out the bleaching in such a way that in concentrated surfactant solutions after a first oxidative step the residual unreacted amount of hydrogen peroxide or oxidizing secondary products formed therefrom (e.g. peroxides) can be made harmless in a simple manner without intermediate removal of the stabilizer originally added.